1. Field of the Invention
The present invention relates to a fuel cell using molten carbonate as an electrolyte, and more particularly, to a molten carbonate fuel cell capable of preventing the leakage of a fuel gas due to deterioration of material by suppressing a high temperature generated at a hot section within a unit cell, thereby improving the reliability thereof.
2. Background of the Prior Art
A fuel cell is a power generating apparatus for converting chemical energy into electrical energy using an electrochemical reaction, and is highlighted as a new electrical energy source, because of being an environmental friendly apparatus and having a high power-generating efficiency. Such a fuel cell has a characteristic of continuously generating the power by supplying a fuel through an oxidation reaction of hydrogen and a reduction reaction of oxygen in the air.
In particular, a molten carbonate fuel cell among fuel cells utilizes molten carbonate as an electrolyte, so that the operation is carried out at a high temperature of 650° C. and the speed of the electrochemical reaction is quick. Contrary to a low-temperature fuel cell, electrode reactions occur when the carbonate electrolyte is molten at a high temperature, thereby generating the power at a relatively high temperature. Since the oxidation-reduction reactions of hydrogen and oxygen do not require a catalyst made of noble metal such as expensive platinum, there are features of facilitating the utilization of a nontoxic gas, such as carbon monoxide, and a coal gas. Another feature is to anticipate a high thermal efficiency above about 80% due to the utilization of electricity and waste heat.
The molten carbonate fuel cell has porous anode/cathode electrodes having a wide surface area for facilitating the smooth procedure of the oxidation-reduction reactions of hydrogen and oxygen. The molten carbonate impregnated in a porous ceramic interposed between the porous anode/cathode electrodes functions as a shield of preventing direct contact between the fuel mainly made of hydrogen and the oxidant made of oxygen, and a passage for guiding carbonate ions (CO32−) produced from an air electrode to a fuel electrode.H2+CO32−→H2O+CO2+2e− (fuel electrode)1/20O2+CO2+2e−→CO32− (air electrode)
However, since a unit cell forming one cell generates a low electromotive force of 1 V, it is no properly to be practically used. Such unit cells are stacked, with conductive separator plates interposed between two adjacent unit cells, to constitute the power generating system.
Specifically, the unit cell includes a pair of porous electrode plates (fuel electrode and air electrode), and an electrolyte plate consisting of alkali carbonate interposed between these electrode plates. These fuel cells are stacked one above another, and a conductive separator plate is interposed between two adjacent unit cells. The separator plate electrically connects these unit cells, and provides the fuel electrode with a passage of a fuel gas and the air electrode with a passage of an oxidant gas.
Such the fuel cell of stacked structure requires a manifold for distributing and collecting the reaction gas. The gas to be required for the reaction is supplied via an inlet manifold, and after passing through the air electrode and the fuel electrode, is outwardly discharged via a manifold opposed to the inlet manifold. Each unit cell is provided with a wet seal formed by molten carbonate, in order to prevent the fuel and the oxidant from being mixed within the fuel cell. The body of the fuel cell and the manifold are also wet-sealed together in order to prevent the reaction gases from leaking out.
In case of the fuel cell, however, a part of energy contained in the fuel is converted into the electrical energy, and the remainder is converted into the atmospheric heat. Accordingly, in case of the stacked fuel cell, the heating value is varied depending upon a degree of the stack. The more the fuel cells are stacked, the more the heating value is generated. Therefore, the hot section is produced at the outlet of the gas.
This high temperature has an influence on the components of the fuel cell, i.e., the electrodes, the electrolyte and the separator plates. Specifically, there are some situations: change of the porous structure, and evaporation of the liquid electrolyte, which are due to the high temperature; consumption of the electrolyte and deformation of the separator plates, which are due to the increased corrosion of the metal separator plate; and leakage of the fuel gas, due to these causes. Therefore, the lifetime of the fuel cell is significantly reduced.
In order to suppress the production of the hot section, a method is widely used to cool it by overly supplying the oxidant gas mainly comprising air. The oversupply of air at the defined passage provides the gas flow with a resistance, thereby causing the pressure to be increased.
The conventional molten carbonate fuel cell isolates the fuel from the oxidant gas by use of the electrolyte impregnated in the porous ceramic in a type of wet seal. However, since the oversupply of the oxidant for suppressing the production of the hot section causes the high pressure to be produced within the passage, the fuel gas is leaked out due to the rupture of the wet seal, thereby significantly shortening the life of the fuel cell body.
Another method is used to reduce the degree of the electrode reactions, i.e., lower the current density, so that the production of the hot section may be suppressed by use of the small heating value. However, the utilization of the lower current density causes the usage of the molten carbonate fuel cell to be highly limited as a power-generating plant having a high output.
One method employing internal and external manifolds in the fuel cell body is disclosed in Japanese Laid-Open Patent Publication No. 62-202465. Intake and exhaust of the fuel gas are distributed at its center by use of the internal manifold, and the fuel gas is exhausted by use of internal manifolds at both sides. The oxidant gas mainly used for cooling due to a lot amount of gases flows from one surface of the stacked fuel cell to the other opposed surface by use of the external manifold. When using a lot amount of oxidant gases in a long passage of oxidant gas, the leakage of the fuel gas due to the pressure generation is not evitable.
Another method, similar to the above-mentioned method, is disclosed in Japanese Laid-Open Patent Publication No. 61-248364. The fuel gas is supplied from internal manifolds provided at both sides of a separator plate, and is collected and exhausted to a center internal manifold, so that the fuel gas and the oxidant gas are perpendicularly crossed. Since the oxidant gas in charge of cooling the fuel cell body flows from one surface of the stacked fuel cell to the other opposed surface, it is difficult to supply a lot amount of oxidant gases using the pressure generated by means of the long passage.